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<article id="post-Mobx6核心源码解析（四）  ObservableValue" class="h-entry article article-type-post" itemprop="blogPost" itemscope itemtype="https://schema.org/BlogPosting">
  
    
   
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        Mobx6核心源码解析（四）: ObservableValue
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          <h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><figure class="highlight ini"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">let <span class="attr">target</span> = observable(obj)</span><br></pre></td></tr></table></figure>

<p>在Mobx中<code>Atom</code>用来监测数据被观察和修改，在设置target的属性描述符的时候，对应每一个key都会创建一个一个<code>Atom</code>的子类<code>ObservableValue</code>实例。</p>
<h2 id="ObservableValue"><a href="#ObservableValue" class="headerlink" title="ObservableValue"></a>ObservableValue</h2><p>如果<code>obserable(obj)</code>的返回值target是”<em>明</em>“,而obj是”<em>暗</em>“，那么<code>ObservableValue</code>实例则是”<em>明</em>“通往”<em>暗</em>“的单向桥梁。对target相应key的getter和setter都调用了这个实例的相应方法。</p>
<ul>
<li>value_ 这个属性保存了obj对象的对应的key的<code>descriptor.value</code></li>
<li>get： 当访问target的属性的时候，会调用这个方法。返回的就是原obj对象对应key的相应value<code>reportObserved</code>，</li>
</ul>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"> <span class="keyword">public</span> <span class="keyword">get</span>(): T &#123;</span><br><span class="line">    <span class="keyword">this</span>.reportObserved()</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">this</span>.dehanceValue(<span class="keyword">this</span>.value_)</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>get的调用说明该key被依赖，其会调用<code>reportObserved</code>处理依赖。</p>
<ul>
<li>reportObserved 这个方法继承自父类<code>Atom</code>,该函数中最有价值的事情是确定依赖关系。</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line">export function <span class="title function_">reportObserved</span><span class="params">(observable: IObservable)</span>: <span class="type">boolean</span> &#123;</span><br><span class="line">    ...</span><br><span class="line">    <span class="type">const</span> <span class="variable">derivation</span> <span class="operator">=</span> globalState.trackingDerivation</span><br><span class="line">    <span class="title function_">if</span> <span class="params">(derivation !== <span class="literal">null</span>)</span> &#123;</span><br><span class="line">       </span><br><span class="line">        <span class="keyword">if</span> (derivation.runId_ !== observable.lastAccessedBy_) &#123;</span><br><span class="line">            observable.lastAccessedBy_ = derivation.runId_</span><br><span class="line">          </span><br><span class="line">            derivation.newObserving_![derivation.unboundDepsCount_++] = observable</span><br><span class="line">            <span class="title function_">if</span> <span class="params">(!observable.isBeingObserved_ &amp;&amp; globalState.trackingContext)</span> &#123;</span><br><span class="line">                observable.isBeingObserved_ = <span class="literal">true</span></span><br><span class="line">                observable.onBO()</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> <span class="literal">true</span></span><br><span class="line">    &#125; <span class="keyword">else</span> <span class="keyword">if</span> (observable.observers_.size === <span class="number">0</span> &amp;&amp; globalState.inBatch &gt; <span class="number">0</span>) &#123;</span><br><span class="line">        queueForUnobservation(observable)</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> <span class="literal">false</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><code>globalState.trackingDerivation</code>是一个<code>IDerivation</code>类型的数据，或者称之为”<em>派生</em>“，它有一个大名鼎鼎的实现类，叫<code>Reaction</code>。可以简单理解：<strong>derivation是observable数据的观察者。</strong> 当这段代码运行的时候就会创建出一个<code>reaction</code>，并挂到<code>globalState.trackingDerivation</code>上，这个过程在解析<code>autorun</code>源码的时候再具体分析。</p>
<figure class="highlight scss"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">auturun</span>(()=&gt;&#123;...&#125;)</span><br></pre></td></tr></table></figure>

<p>总之，这里通过<code>derivation.newObserving_![derivation.unboundDepsCount_++] = observable</code>,确定了<code>derivation</code>对这个<code>ObservableValue</code>实例有依赖。</p>
<ul>
<li>set和setNewValue 当对target设置&#x2F;修改值的时候会调用<code>set</code>或<code>setNewValue</code>，set内部也调用了<code>setNewValue</code>.</li>
</ul>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">set</span>(newValue: T) &#123;</span><br><span class="line">    <span class="keyword">const</span> oldValue = <span class="keyword">this</span>.value_</span><br><span class="line">    newValue = <span class="keyword">this</span>.prepareNewValue_(newValue) <span class="keyword">as</span> any</span><br><span class="line">    ...</span><br><span class="line">    <span class="keyword">if</span> (newValue !== globalState.UNCHANGED) &#123;</span><br><span class="line">        <span class="keyword">this</span>.setNewValue_(newValue)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">setNewValue_(newValue: T) &#123;</span><br><span class="line">    <span class="keyword">const</span> oldValue = <span class="keyword">this</span>.value_</span><br><span class="line">    <span class="keyword">this</span>.value_ = newValue</span><br><span class="line">    <span class="keyword">this</span>.reportChanged()</span><br><span class="line">   ...</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><code>setNewValue</code>触发时说明value变更，需要通知依赖进行相应的处理，这里调用的是<code>reportChanged</code>。</p>
<ul>
<li>reportChanged 这个方法也继承自父类<code>Atom</code></li>
</ul>
<figure class="highlight scss"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">public <span class="built_in">reportChanged</span>() &#123;</span><br><span class="line">       <span class="built_in">startBatch</span>()</span><br><span class="line">       <span class="built_in">propagateChanged</span>(this)</span><br><span class="line">       <span class="built_in">endBatch</span>()</span><br><span class="line">   &#125;</span><br></pre></td></tr></table></figure>

<p><code>propagateChanged</code>从<code>ObservableValue</code>实例<code>observable</code>中找到对它有依赖的所有<code>derivation</code>，并调用<code>onBecomeStale_</code>,这个方法在<code>Reaction</code>中定义，它在经过了一系列复杂的调用后，能够使得依赖的函数再次运行。</p>
<figure class="highlight ini"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">export function propagateChanged(observable: IObservable) &#123;</span><br><span class="line">    // invariantLOS(observable, &quot;changed start&quot;)<span class="comment">;</span></span><br><span class="line">    if (<span class="attr">observable.lowestObserverState_</span> === IDerivationState_.STALE_) return</span><br><span class="line">    <span class="attr">observable.lowestObserverState_</span> = IDerivationState_.STALE_</span><br><span class="line"></span><br><span class="line">    // Ideally we use for..of here, but the downcompiled version is really slow...</span><br><span class="line">    observable.observers_.forEach(<span class="attr">d</span> =&gt; &#123;</span><br><span class="line">        if (<span class="attr">d.dependenciesState_</span> === IDerivationState_.UP_TO_DATE_) &#123;</span><br><span class="line">            if (__DEV__ &amp;&amp; d.isTracing_ !== TraceMode.NONE) &#123;</span><br><span class="line">                logTraceInfo(d, observable)</span><br><span class="line">            &#125;</span><br><span class="line">            d.onBecomeStale_()</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="attr">d.dependenciesState_</span> = IDerivationState_.STALE_</span><br><span class="line">    &#125;)</span><br><span class="line">    // invariantLOS(observable, &quot;changed end&quot;)<span class="comment">;</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="未完待续"><a href="#未完待续" class="headerlink" title="未完待续"></a>未完待续</h2><p>Mobx中“观察”部分的代码到这里就基本结束了，但是遗留了两个问题：<code>globalState.trackingDerivation</code>是何时确定的以及<code>observable.observers_</code>又是如何收集的。</p>
<p>下一节将开始Mobx“反应”部分的源码解析，回答这两个问题</p>

        
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